Testosterone secretion is the net result of intratesticular testosterone biosynthesis, degradation and finally, its release into the testicular vasculature. While considerable information is available regarding testosterone biosynthesis, little is known about intratesticular testosterone degradation. In comparison to our knowledge of the cellular secretion of proteinaceous molecules, virtually nothing is known about the mechanism by which steroids are released into the vasculature of the steroid secreting organ. The testis is no expception to this rule. The rabbit testis perfused in vitro is being used to investigate the secretion and biological activity of testicularly derived testosterone metabolites such as dihydrotestosterone and estrogens; the mechanism by which testosterone escapes into the testicular vasculature from the steroidogenic cell and the potential regulatory effects of testicular steroids on testosterone secretion.